Sleep apnea (SA) is a breathing disorder characterized by repetitive complete or partial cessations of breathing (apneas and hypopneas, respectively) during sleep. The frequency of these events ranges from 5 to 100 times/hour depending on the severity of the case. As a result, patients suffer from poor sleep quality, daytime sleepiness, and poor cognitive performance. Sleep apnea can generally be characterized as one of two types—obstructive and central sleep apnea (OSA and CSA, respectively). It has been observed that OSA, which is the most common type, increases the risk of developing hypertension, heart failure (HF), and stroke by 3 to 4 fold. Also, patients with untreated sleep apnea generally consume twice as many healthcare resources for treatment of cardio-respiratory diseases than subjects without the disease. On the other hand, it has been demonstrated that treating OSA in patients with hypertension or HF lowers blood pressure, and dramatically improves cardiovascular function. Therefore, diagnosing and treating such patients could have a very substantial beneficial medical and public health impact. Unfortunately, the majority of people with sleep apnea remain undiagnosed due to the lack of accessibility to expensive overnight monitoring in a sleep laboratory presently required for diagnosis.
Obstructive sleep apnea (OSA) is generally understood to result from partial or complete collapse of the pharynx or the upper airway (UA) resulting in obstruction of the airflow pathway. In OSA, the respiratory drive is still present but the patient is breathing against a high resistance tube—a situation that mimics chocking. Thus, the hallmark of OSA is narrowing, obstruction, or total closure of the upper airway (pharynx). This results in characteristic breath sounds such as the occurrence of snoring and turbulent sounds. Each event generally lasts 10 to 60 seconds, thus generally causing episodes of oxygen deprivation and often provoking arousals from sleep and consequent sleep fragmentation. As a result, patients suffer from poor sleep quality, daytime sleepiness, and impaired cognitive performance. It is a common disease affecting approximately 7% of adults. Nevertheless, the majority of patients with OSA remain undiagnosed; in one study, it was shown that 93% of women and 82% of men with moderate to severe OSA had not been diagnosed.
Central sleep apnea (CSA), on the other hand, is generally understood to occur when there is a temporary cessation of respiratory output from the respiratory neurons in the brainstem to the muscles of respiration. This lack of respiratory muscle activation causes a temporary cessation of airflow (i.e. central apnea), during which there is no respiratory ventilation. In contrast to OSA, the upper airway is usually open during CSA, and thus chocking sounds and snoring are less likely to occur. Further, when airflow resumes, snoring does not necessarily occur because the pharynx is usually not obstructed.
The distinction between CSA and OSA can be of particular importance in choosing the management of the sleep apnea and associated diseases. This is especially important in patients with heart failure (HF) or stroke in whom CSA is common and is associated with increased mortality risk. Patients with HF have a very high prevalence of both OSA and CSA. The distinction is important for choosing the appropriate therapy. For example, in OSA, therapy usually consists of Continuous Positive Airway Pressure (CPAP), whereas in CSA the treatment strategy is generally to first treat the underlying HF, and if CSA persists, to use adaptive servo ventilation, oxygen or CPAP. It has also been shown that suppression of CSA by CPAP in HF patients improves the cardiovascular function, and tends to improve survival.
Presently, the standard means of identifying and diagnosing sleep apnea is via overnight polysomnography (PSG), in which the patients have to sleep in a laboratory attached to many monitoring electrodes under the supervision of a technician. PSG is expensive and access to it is limited, resulting in long waiting lists in the limited areas where PSG is available.
For this reason, interest has been raised in devising new methods to diagnose sleeping disorders, such as SA. For example, acoustic analysis of respiratory sounds has gained an increasing role in the study of respiratory disorders such as in identifying pathological respiratory sounds including wheezes and crackles, and to study and locate the site of snoring. In some sleep studies, snoring sounds were captured above the mouth level, as were tracheal sounds, to study snoring, particularly as snoring is a component of the disease itself and is produced at the very location where narrowing and obstruction takes place.
Despite recent findings, snore-driven techniques have fundamental limitations from the clinical perspective. For instance, snoring does not necessarily occur in all types of SA, such as in CSA. Furthermore, snore-driven techniques generally fail to assess the severity of an identified condition. For example, while snoring is a hallmark of OSA, it might not necessarily take place with each apnea and hypopnea. Accordingly, assessing the disease severity in terms of frequency of apneas per hour might be underestimated if some apneas are missed due to absence of snoring, for example. As knowledge about the disease severity can be beneficial in selecting an appropriate treatment strategy, snore-driven techniques can be less than ideal.
Accordingly, while some work has been done to detect the occurrence of OSA from snoring sounds, there remains much room for improvement, be it in the development of a reliable technique for differentiating between OSA and CSA occurrences, or identifying CSA altogether, and/or for evaluating the severity of such occurrences. Demand is also increasing for reliable apnea identification, characterization and/or diagnostic techniques that can be accessed by a wider base of the population, for example as compared to the technician-assisted PSG techniques currently implemented in dedicated sleep laboratories.
Therefore, there remains a need for new breathing disorder identification, characterization and diagnosis methods, devices and systems that overcome at least some of the drawbacks of known techniques, or at least, provides the public with a useful alternative.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the invention.